The present invention relates generally to fluid ejection devices, and more particularly to self-calibration of power delivery control to firing resistors in fluid ejection devices.
One type of conventional fluid ejection system is a conventional inkjet printing system which includes a printhead, an ink supply which supplies liquid ink to the printhead, and an electronic controller which controls the printhead. The printhead ejects ink drops through a plurality of orifices or nozzles and toward a print medium, such as a sheet of paper, so as to print onto the print medium. Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium as the printhead and the print medium are moved relative to each other.
Typically, the printhead ejects the ink drops through the nozzles by rapidly heating a small volume of ink located in vaporization chambers with small electric heaters, such as thin film resistors. Heating the ink causes the ink to vaporize and be ejected from the nozzles. Typically, for one dot of ink, a remote printhead controller typically located as part of the processing electronics of a printer, controls activation of an electrical current from a power supply external to the printhead. The electrical current is passed through a selected thin film resistor to heat the ink in a corresponding selected vaporization chamber. The thin film resistors are herein referred to as firing resistors.
Typically, a high-current load on the power supply supplying the electrical current to the firing resistors occurs if a large number of firing resistors are simultaneously energized on a single printhead die. The resulting high electrical current flowing through parasitic resistances in conductors to the printhead die causes the voltage at the printhead die to sag. Less energy is delivered to the firing resistors as a result of this voltage sag at the printhead die.
In one conventional inkjet printing system, large by-pass capacitors are disposed adjacent to the printhead to alleviate a portion of this voltage sag. Nevertheless, any resistance between the large by-pass capacitors and the printhead is not compensated for in this conventional inkjet printing system. Furthermore, a DC sag on the power supply supplying the electrical current to the firing resistors under continuous load is also not compensated for in this conventional inkjet printing system.
In one conventional inkjet printing system, the duration of the power being supplied to the firing resistors is modulated in response to a change in the power supply voltage at the printhead. In this conventional inkjet printing system, constant energy is delivered to each firing resistor. Nevertheless, firing resistors receive more instantaneous power when only a few firing resistors are energized. The life of a firing resistor can be increased by reducing the amount of instantaneous power delivered to the firing resistor. Therefore, there is a desire to have both a fixed power applied to the firing resistors and a fixed duration that the fixed power is applied to the firing resistors.
For reasons stated above and for other reasons presented in the Detailed Description section of the present specification, a fluid ejection device, such as an inkjet printhead, is desired which minimizes instantaneous power delivered to firing resistors to thereby increase the life of the fluid ejection device. In addition, there is a desire for a fluid ejection device which minimizes instantaneous power delivered to firing resistors and has a simplified calibration process where the calibration time of the fluid ejection device is not significantly increased by the instantaneous power delivery minimization technique.
One aspect of the present invention provides a fluid ejection device including an internal power supply path and a power regulator providing an offset voltage from the internal power supply path voltage. The power regulator includes a self-calibration circuit adapted to determine a regulation band of the power regulator defined by a lower set point offset voltage and an upper set point offset voltage. The fluid ejection device includes a group of nozzles, a corresponding group of firing resistors, and a corresponding group of switches. The switches are controllable to couple a selected firing resistor of the group of firing resistors between the internal power supply path and the offset voltage to thereby permit electrical current to pass through the selected firing resistor to cause a corresponding selected nozzle to fire.